The present invention relates generally to inflatable vehicle occupant restraint systems (commonly known as xe2x80x9cairbagsxe2x80x9d) and more specifically to ignition enhancers and hybrid inflator gas heaters. More particularly, the enhancer compositions disclosed herein use tetrazoles and oxidizers in combination with specified metals to provide reliable ignition of a main gas generant charge. The enhancer compositions are also very effective as gas heaters in hybrid inflator systems as a result of the high combustion temperatures of the inventive compositions. The ignition enhancers disclosed herein are low in cost, provide pre-pressurization of the airbag inflator system to reliably ignite the gas generant compositions and efficiently heat the pressurized gas in a hybrid inflator system.
Gas generating compositions are useful in a number of different applications. One important use for such compositions is in the operation of airbags. Airbags are gaining in acceptance to a point that many, if not most, new motor vehicles are equipped with such devices. Many new motor vehicles are equipped with multiple airbags to protect the driver and passengers.
A sufficient volume of gas must be generated in a motor vehicle airbag system to inflate the bag within a fraction of a second. Between the time the motor vehicle is impacted in a crash and the time the occupant would otherwise impact against the steering wheel, door or dashboard, the airbag must fully inflate. As a consequence, rapid gas generation is mandatory.
Both organic and inorganic materials have been proposed as possible gas generants. Such gas generant compositions include oxidizers and fuels which react at sufficiently high rates to produce large quantities of gas in a fraction of a second. At present, sodium azide is the most widely used and currently accepted gas generating material. However, numerous alternatives have been proposed to the sodium azide-based generants which overcome a number of problems associated with sodium azide (i.e., toxicity, cost, safety of handling and disposal concerns).
In addition to airbag systems based solely on gas generating compositions, hybrid inflator technologies have also been developed. Hybrid inflators generally require the heating of a stored, inert gas (i.e., argon or helium) to a desired temperature by igniting a small amount of a rapidly burning gas generating composition. Hybrid gas generating systems comprise a pressure vessel having a rupturable opening, a pre-determined amount of inert gas disposed within the pressure vessel, a gas generating composition for producing hot combustion gases and a means for rupturing the rupturable opening. The gas generating composition is configured and positioned relative to the pressure vessel so that hot combustion gases are mixed with and heat the inert gas. The mixed and heated gases exit the pressure vessel through an opening and ultimately exit the inflator to inflate an inflatable object.
In conventional pyrotechnic airbag inflators or in hybrid inflators, a combustion chain or series of combustion events is used to result in the inflation of the airbag. The combustion is begun by an ignition initiator, preferably an electrically activated squib which contains a small charge of an electrically ignitable composition. However, it is understood that any suitable ignition initiator, such as a stab initiator, may be used to practice the present invention. The ignition initiator is connected to at least one remote crash sensing device of a type well known in the art. The crash sensing device may be located, for example, in the front bumper and/or side fenders of a motor vehicle. As presently used in motor vehicles the squib with the electrically ignitable composition abuts against an enhancer packet or chamber containing a quantity of an ignitable enhancer composition such as a mixture of boron and potassium nitrate (BKNO3). The enhancer composition is typically employed in powdered form to provide the maximum available burning surface for the fastest possible response, thereby rapidly igniting the main gas generating charge.
The present invention is directed to the discovery of an enhancer composition that can be used to replace the expensive and dangerous BKNO3. Further, the enhancer composition of this invention provides reliable ignition of the main gas generant charge and the pre-pressurization of the inflator vessel so as to enhance the performance of the main gas generant composition. An unexpected advantage of this invention is that the enhancer composition is not required to be used in powdered form, allowing enhancements from production and safety perspectives. Additionally, the enhancer composition of the present invention reliably ignites the main gas generant composition over a wide temperature range, whether it be azide or non-azide based.
Thus, in operation of a preferred embodiment, a signal from a crash sensor results in the initiation of a charge within the squib which then ignites a charge within the body of the squib. A stream of hot gases and particles produced by this combustion is then directed into the enhancer composition disclosed herein whereupon the enhancer composition begins to burn. The rapid generation of hot gases and molten reaction products produced by the combustion of the enhancer composition impinges upon the main gas generant composition (sometimes referred to in the art as a xe2x80x9cpropellantxe2x80x9d) which itself begins to burn.
Typically, the main gas generant mixture is in the form of pellets or wafers. Preferably, the enhancer composition is also in the form of pellets. An appropriate amount of gas generant (enhancer plus main charge), calculated to produce an appropriate amount of gas to inflate the attached airbag within ten to eighty milliseconds, is placed within the gas generant vessel.